In semiconductor fabrication, it is desirable to employ automated handling of the semiconductor slices for efficient processing of the slices. Normally, the slices are contained in a carrier cassette which holds a stack of semiconductor slices. During the automated processing of the semiconductor slices, the slices are removed from and replaced into the cassette. While being moved, the slices must be held securely, but gently, and must not interfere with adjacent slices in the cassette. Preferably, the handling apparatus should minimize contact with the surfaces of the slice.
Previously developed slice handling equipment generally use an elevator mechanism on the cassette to present the semiconductor slice to the handling apparatus. Typically, the handling equipment comprises an "O" ring belt or "air track" slice transfer mechanism. These mechanisms require the lowest slice to be removed from the cassette first, and the highest slot in the receiving cassette to be loaded first. Also, the slice cannot be processed with the active surface face down as desired.
Robotic handling of slices, as opposed to "O" ring belt or "air track" handling, requires the use of an end effector. Such end effectors suffer from several deficiencies. First, when the slices are handled in a vacuum, the slice is normally held in a partial cavity to limit side motion of the slice during transit, necessitating the loading of the slice into the end effector with the slice's active surface (surface undergoing processing) facing upward; otherwise, the edges of the slice would be damaged during transfer. However, recent studies have shown that processing with the active surface of the slice facing downward prevents contamination by undesirable particles in the chamber during processing. Therefore, the previously developed slice handling equipment is not operable to hold the slice in the most desirable position.
Second, the end effectors require a large cleared space directly beneath the slice, mandating that only the bottom slice in the cassette be removed. Thus, the handling equipment was not able to extract a single slice from any position within the cassette, without disturbing the slice below.
Third, slice handling end effectors normally use a vacuum as the holding force. This requires hoses and tubes that restrict motion of the handling apparatus. Also, these end effectors are not suitable for processing within a vacuum where the pressure differs, and thus wherein the holding force would be zero.
Thus, a need has arisen in the industry for a slice holding apparatus operable to remove and reinsert individual slices from a cassette, without damage to the semiconductor slices or to other slices within the cassette. The handling apparatus should be operable to hold the semiconductor slice with its active surface down, while maintaining a firm grip on the slice.